Modern wireless communication, radar and radio frequency identification (RFID) systems often operate under full duplex operation. A wireless transceiver comprises of a local transmitter and a local receiver. Full duplex operation occurs when a local transmitter is actively transmitting RF signals during the same time that a local receiver is detecting RF signals and/or backscatter from the surrounding environment. The local transmitter and local receiver are typically in close proximity to one another and are often placed within a common enclosure. It is also desired to operate the full duplex system using a monostatic configuration, namely a configuration that uses a single antenna common to both the local transmitter and local receiver. In a typical transceiver, the transmitted and received signals are typically routed to and routed from the single antenna using a duplexing filter, circulator or directional coupler.
It is known that the operation of a local receiver during the time that a local transmitter is transmitting creates receiver problems as the transmitter energy leaks, couples and/or reflects into the receiver resulting in corruption, distortion, saturation and/or desensitization within the receiver. In some cases, a duplexing filter may be used to isolate the transmitted energy from the receiver if the transmitter and receiver are configured to operate at two different RF carrier frequencies that allow the duplexing filter to provide the required isolation between the transmitter and receiver. If the system is designed to operate with the transmitter and receiver using the same RF carrier frequency or with different transmit and receive frequencies that are close in RF carrier frequency such that the duplexing filter can not adequately provide the required isolation, then a circulator or directional coupler is typically used to isolate the transmitted signal from entering the receiver. Depending on the isolation performance of the circulator or coupler, the system performance may degrade when a portion of the transmitted energy leaks into the receiver.
Circulators and directional couplers are three and four port devices that are used to route RF and microwave signals between various ports within the component. A RF or microwave signal entering the circulator or coupler is expected to exit at a desired port(s) where one port is isolated from the incident signal. In practice, a portion of the incident signal leaks or couples to the isolated port. The ratio of the undesired leakage to the incident signal is often referred to as the isolation of the device. When referring to directional couplers, the isolation term is sometimes referred as the coupler directivity which is defined as the (dB) difference between the isolation and the coupling value of the directional coupler.
A basic circulator, 1, is a three-port device that provides primary signal transmission between pairs of ports. A symbolic diagram of a circulator 1 is shown in FIG. 1. Signals are routed between pairs of ports in the direction of circulation arrow 5. For this example, the circulation arrow 5 in FIG. 1 shows a clockwise direction for signal paths. This circulation arrow 5 is used in the technical literature as a symbolic reference to the direction of signal paths within the circulator. Circulators can be manufactured to have either clockwise or counter-clockwise signal directions. A signal 6 entering the input port 2 will exit through the desired output port 3 following the clockwise circulation arrow 5. Ideally, the signal 7 leaving the circulator 1 will have the same magnitude level as the input signal 6. In practice, the signal 7 leaving the circulator 1 will have some reduction in amplitude due to losses and mismatches that occur inside the circulator. The signal 7 leaving the circulator 1 will also have a phase shift relative to the input signal 6. Ideally, no portion of the input signal 6 should leave the third port 4. This third port 4 is the isolated port. In practice, the isolated port 4 will have a signal level 8 reduced by approximately 20 dB when measured relative to the input signal 6. In practice, junction circulators typically have a minimum isolation of 20-25 dB and lumped element circulators typically have a minimum isolation of 13 dB.
The circulator 1 can also be configured to operate with the input signal entering port 3 and exiting through port 4. In this case port 2 is the isolated port. The circulator 1 can also be configured to operate with the input signal entering port 4 and exiting through port 2. In this case port 3 is the isolated port.
A typically transceiver application using a monostatic antenna configuration is shown in FIG. 2. In the case a local transmitter 9 generates a transmitted signal 6 that enters the input port 2 of the circulator 1. The circulator 1 routes the signal to the common port 3. The signal 7 leaves the circulator at the common port 3 and enters the antenna 12. Any received signal 10 captured by the antenna 12 from the surrounding environment enters the common port 3 of the circulator 1 and is routed to the output port 4. The desired received signal 13 leaving the output port 4 enters the local receiver 11. If the system uses a local transmitter 9 and local receiver 11 that are operating simultaneously, then a portion of the signal 6 from the active transmitter 9 may couple or leak through the circulator 1 and enter the active receiver 11. This undesired coupled signal 8 may reduce the performance of the receiver 11.
For certain applications where additional losses in the receive path will not effect the required system performance, it is possible to replace the circulator with a directional coupler. In this configuration, the directional coupler is used to route the signals between the transmitter 9 to the antenna 12, and from the antenna 12 to the receiver 11. FIG. 3 shows a monostatic antenna configuration using a directional coupler 21. In this configuration, the directional coupler 21 is positioned in order to transfer the signal 6 emitted from the local transmitter 9 to the antenna 12. The directional coupler is typically a four-port device where one of the ports are terminated using a resistive termination 25. The termination 25 is often matched to the characteristic impedance of the system, which is typically 50 ohms. In some cases, the termination 25 is included internally to the directional coupler which effectively makes the device into a three port component. A three port description for the directional coupler will be used throughout this disclosure. In the configuration shown in FIG. 3, the termination 25 is used to absorb energy coupled from the incident signal 6. Ideally no portion of the incident signal 6 should leave port 24 which connects the directional coupler 21 to the receiver 11. For signal reception from the surrounding environment, any received signals 10 captured by the antenna 12, will enter the directional coupler 21 at common port 23. A portion of the received signal 10 will be coupled to the output port 24. The desired coupled signal 13 will then enter the receiver 11. As the desired signal 13 is coupled to the output port 24 the amplitude level of the desired signal 13, will be reduced by the coupling factor. In some full duplex systems, such as passive UHF RFID systems, this additional loss in received energy does not create difficulties when recovering the received information as these systems are generally forward link limited. Problems may occur when a portion of the incident transmitter signal couples or leaks into the receiver through the directional coupler which in turn may reduce receiver performance. In this case, a portion of the transmitted signal, 6, may couple or leak through the directional coupler 21 and exit the output port 24. This undesired coupled signal 8 will enter the receiver 11 and may reduce the performance of the receiver 11.
In both configurations, shown in FIG. 2 and FIG. 3, it is important that the receiver not be desensitized by any signal(s) coming from the system. Signals that could desensitize the receiver include signals received by the antenna and signals that leak or couple over from the transmit channel. If the signal received by the antenna from the surrounding environment is the signal of interest, then it is assumed that the system has been designed as not to desensitize the receiver when this signal is present. Therefore undesired receiver desensitization may occur when signals leak from the local transmitter into the local receiver. It is well known in industry, that fabricating a circulator with very high isolation (>30 dB) is often difficult and expensive. A typical junction circulator may have 20 dB isolation resulting in 1% of the transmit energy leaking into the receiver channel. This leakage signal may greatly affect the performance of the receiver in a full duplex system. As an example, a typical RFID system that uses a transmitter with an output power of 1 watt (+30 dBm) and a circulator with an isolation of 20 dB would have an unwanted signal entering the receiver of 10 mwatt (+10 dBm). This level of undesired signal would typically saturate and/or desensitize a low power mixer placed in the front end of the receiver.